1. Field of the Invention
The present invention relates to a load simulator used in a plasma processing system or the like.
2. Description of Related Art
Recent years have seen the development of a plasma processing system in which a workpiece such as a semiconductor wafer or a liquid crystal substrate is processed using a method such as etching. FIG. 8 is a block diagram showing the configuration of a general plasma processing system. The plasma processing system shown in FIG. 8 includes a high frequency power source apparatus 100, an impedance matching apparatus 200, a high frequency measurement apparatus 300, and a plasma processing apparatus 400. The high frequency power source apparatus 100 outputs high frequency power, which is supplied to the plasma processing apparatus 400.
The high frequency measurement apparatus 300 monitors the impedance and the like of the plasma processing apparatus 400 during plasma processing. The high frequency measurement apparatus 300 is connected to an input terminal of the plasma processing apparatus 400 and includes sensors for detecting the high frequency voltage and the high frequency current at the input terminal. The high frequency measurement apparatus 300 also calculates various types of high frequency parameters for the impedance and the like based on the detected values of the high frequency voltage and the high frequency current.
In general, with high frequency measurement apparatuses, the detected values obtained by the sensors can deviate from correct values due to errors in the sensor sensitivity and the like. In light of this, a measurement object serving as a reference is measured by the measurement apparatus in advance, and a calibration parameter is acquired based on the measurement results.
In actual measurement, the detected values obtained by the sensors are converted into correct values using the calibration parameter, and the converted values are output (e.g., see JP 2004-309132A and JP 2007-163308A).
With the high frequency measurement apparatus 300 shown in FIG. 8, SOLT (Short-Open-Load-Thru) calibration, for example, is used for correction of the high frequency voltage and the high frequency current. In SOLT calibration, a load simulator (dummy load) 500 (see FIG. 9) for which the real impedance value has been specified in advance is connected to the high frequency measurement apparatus 300, and impedance measurement is performed by the high frequency measurement apparatus 300. Three types of load simulators having mutually different impedances are used when performing this measurement. Specifically, the load simulators that are used are a load simulator having the characteristic impedance of the measurement system (the characteristic impedance of the transmission line that transmits a high frequency for measurement, which is generally 50Ω or 75Ω), a load simulator having the open condition impedance (substantially infinite), and a load simulator having the short circuit condition impedance (substantially zero). Next, a calibration parameter for correcting the high frequency voltage and the high frequency current is calculated from the measured impedance values of the load simulators that were measured by the high frequency measurement apparatus 300 and the real impedance values of the load simulators, and the calibration parameter is recorded in a memory of the high frequency measurement apparatus 300.
In the actual measurement, the detected high frequency voltage and high frequency current are corrected using the calibration parameter recorded in the memory, and various types of high frequency parameters are calculated based on the corrected values.
With the calibration method described above, the high frequency measurement apparatus 300 is directly connected to each of the load simulators 500 when the calibration parameter is calculated. The calibration parameter is therefore for correcting the values at the output terminal of the high frequency measurement apparatus 300. The impedance that is corrected based on such a calibration parameter is the impedance obtained when the load side is viewed from the output terminal of the high frequency measurement apparatus 300. However, monitoring the plasma processing apparatus 400 requires measurement of the impedance between electrodes provided inside the chamber of the plasma processing apparatus 400.
In the case where the high frequency measurement apparatus 300 and the plasma processing apparatus 400 are directly connected to each other as shown in FIG. 8, the impedance obtained when the load side is viewed from the output terminal of the high frequency measurement apparatus 300 can be thought to correspond to the impedance between the electrodes in the plasma processing apparatus 400. However, the precision of the measured value obtained by the high frequency measurement apparatus 300 decreases since these two impedances are not completely the same.